With a growth of data traffic and an increase of a service quality requirement, a single physical link in a data link layer cannot satisfy an operating requirement on both the bandwidth and the reliability aspects, while the Link Aggregation Control Protocol (LACP) solved this problem very well. The LACP virtualizes two or more physical links to one logical link, and the logical link, as a whole, is transparently presented to an application of an upper layer. The bandwidth of the logical link is a sum of the bandwidths of all workable physical links. The physical ports which are bound to an identical logical link should have the same Ethernet link layer characteristics, such as a port speed, duplexing, a port state, etc. The advantage of doing it like this is that, when a certain physical link therein is down, that is, fails, the load of the services carried by that physical link can be shared by other physical links. As long as there are certain amounts of available physical links in the logical link, the upper layer service will not be affected. The LACP technology has increased the bandwidth of the link and realized the redundant backup of the link.
Between two interconnected devices, the LACP performs a negotiation through sending the LACP data unit (LACPDU) based on the physical link, wherein, the LACPDU includes the information of the local terminal (Actor) and the opposite terminal (Partner). The LACP runs a standard state machine based on the LACPDU information interacted by the two devices, while it does not run the standard state machine based on commands. The LACP selection state machine selects the physical link according to the local terminal and the opposite terminal information included in the LACPDU.
The LACP selection state machine manages the physical links with the same characteristic parameters through Aggregators, each Link Aggregation Group (abbreviated as LAG) includes at least one Aggregator, each Aggregator has its own characteristic parameters, the characteristic parameters of each Aggregator are different from each other, and the physical link can only be added into an Aggregator with the same characteristic parameters. When the characteristic parameters of a physical link are changed, the physical link must be removed from the current Aggregator, and then be added into an Aggregator with the same characteristic parameters. The selection parameters which influence the full duplex physical link to select the Aggregator include: a system ID of the local terminal (a system priority and a system Media Access Control (MAC) address), an operation KEY of the local terminal, a system ID of the opposite terminal, an operation KEY of the opposite terminal, and a full duplex mode.
The LACP selection state machine compares the system priorities at first, then compares the system MACs, and finally compares the parameters involving the system in the operation KEY value. If the numerical values thereof are different, they cannot be added into an identical Aggregator. As to the identical system, the LACP selection state machine continues to compare the port states or the port parameters of the physical link involved in the operation KEY value, such as the duplex mode and the bandwidth of the physical link. Generally speaking, the physical links of different duplex modes or different bandwidths cannot be added into an identical Aggregator.
The MAC address of the Ethernet device is made up of 6 bytes. The previous 3 bytes are called OUI, which are registered to the network device manufacturer by the IEEE organization, and each manufacturer has one or more sections in the OUI, which differ from each other; the last three bytes are assigned by the network device manufacturer to each device with the MAC address which is produced by their own, which are different from each other.
It is specified in the IEEE802.3AD standard that the butt jointed devices need to be identified by the system ID, the system ID is consisted of the system priority and the system MAC address of the device, and beyond that, the operation KEY value also includes some system information. The information will also influence the selection state machine, and then influence the selection of the device. Different devices are distinguished by different system IDs in the IEEE802.3AD standard.
It is specified in the IEEE802.3AD standard that all the LACPs are configured between two devices, and do not support multi-device aggregation, so the LACP can only perform the physical link level redundant protection or board card level redundant protection, while cannot perform the device level redundant protection.
As shown in FIG. 1, a Customer Edge (CE) is butt jointed with a Provider Edge (PE1) and a PE2 respectively, and interacts with them by adopting the standard LACP. The system MACs of the PE1 and the PE2 are different. An aggregator is formed between the CE and the PE1, and another aggregator is formed between the CE and the PE2. At a certain moment, only one aggregator is in an active state, and another aggregator is in an inactive state. The CE has priority to butt joint with the PE with the higher system ID priority successfully. The PE with the higher system ID priority is an active one and the PE with lower system ID priority is a backup one, to form a traditional LAG dual-machine backup.
Therefore, the limitation about the system ID in the IEEE802.3AD standard results in that, by adopting the traditional LAG dual-machine backup mechanism, at the same moment, only one PE can be aggregated with the CE successfully, and many PEs can not be used for forwarding simultaneously, which reduces the utilization rate of the device; meanwhile, during the time from the PE which performs forwarding becoming abnormal to it switching to other PE and forwarding normally, all flows will be suspended, and the device level redundant ability is bad.